Development of a vaccine to prevent, or reduce the rate of, HIV infections remains a high priority despite recent setbacks in the field. The lessons from failed and successful experimental programs indicate the need to apply new approaches to HIV vaccine design with the goal of inducing immune responses that are the appropriate type, quality, magnitude and active in the appropriate sites in the body. A promising approach is the use of the Adenovirus serotype 4 (Ad4) as a vaccine delivery vehicle. The Ad4 virus is a component in the US Military adenovirus vaccine which was formulated for administration in an oral dosage form. Oral delivery should be advantageous for HIV vaccines because this route of administration is more likely to induce mucosal immune responses than parenteral injection and would target the gut mucosal tissues in particular. The Ad4 vaccine vector is replication-competent in humans which should drive the induction and expansion of immune responses that are higher in quality, in terms of magnitude and effector functions, than those induced by non-replicating vectors. In Y1, multiple Ad4 vectors will be engineered to express unique antigens including: 1) HIV-1 Env clade C protein with the appropriate modifications for the purpose of inducing antibody responses broadly effective against a variety of HIV strains; and 2) GB virus type C E2 protein, which may induce synergistic antibody responses which would significantly broaden HIV-1 neutralization. In Y1-Y2, all vectors will be assessed for immunogenicity in small animals (rabbits). HIV-1 Env-specific antibody, both neutralizing and binding, will be determined. Completion of this SBIR program will provide sufficient data to determine the utility of this Ad4 vector system for inducing effective antibody and T cell responses and potentially could yield an experimental vaccine suitable for clinical development.